1. Field of the Invention
The invention is related to instruments and techniques for controlled application of energy to tissue, and more particularly relates to supercavitating surfaces and electrosurgical surfaces for ablating tissue layers, for ablating holes in soft tissue and for ablating occlusive materials, calculi and the like.
2. Description of the Related Art
Various electromagnetic and acoustic energy delivery sources have been investigated for surgical tissue ablation or removal, including radiofrequency (Rf) energy delivery, high intensity focused ultrasound (HIFU) tissue interactions and microwave energy absorption in tissue. In general, at high intensities, the above listed energy sources generate thermal effects that can vaporize tissue as the means of tissue ablation or removal. In other words, the energy sources elevate the temperature of water in intra- and extracellular spaces to above 100° C. thereby explosively vaporizing water to damage or destroy the tissue. The drawback to such purely thermally-mediated ablations is significant collateral damage to tissue volumes adjacent to the targeted site. While in many surgical fields the above-described collateral thermal damage may be acceptable, in fields in which thin layer ablations are required such as ophthalmology, neurology and interventional cardiology, there is a need to prevent, or limit, any such collateral damage.
Radiofrequency currents in tissue have been known for many years in the prior art for cutting a tissue mass or for coagulating regions within a tissue mass. Conventional electrosurgical systems known in the art ablate tissue by applying an electrical field across the targeted tissue. The actual energy-tissue interaction in Rf cutting is typically described in terms of a voltage differential that first boils a fluid and then causes a spark or arc across a vapor gap between an active electrode and the targeted site (e.g., coupled to a return electrode). Conventional electrosurgical ablation is generally achieved at frequencies ranging from 500 kHz to 2.5 MHz, with power levels ranging from 75 to 750 W. In such prior art tissue cutting with Rf currents, the current density rapidly decreases with distance from the exact energy deposition site on the tissue which is contacted by the spark. Still, the depth of tissue disruption and damage in such prior art electrosurgical cutting may range from about 0.3 mm. to as much as 3.5 mm. (see R. D. Tucker et al, Histologic characteristics of electrosurgical injuries, Journal Am. Assoc. Gynecol. Laparoscopy 4(2), pp. 201-206 1997.) The depth of tissue ablation depends on several variables, including (i) the conductivity of the tissue, (ii) the insulative characteristics of the media in the physical gap between the active electrode(s) and the tissue; (iii) the dimension of the physical gap between the electrode(s) and the tissue; (iv) the power setting and optional feedback control of the power level based upon electrical characteristics of the targeted tissue; (v) and the translation of the working end relative to the tissue.
One prior art system in the field of electrosurgical ablation was invented by Eggers et al and is described as a Coblator™ (see. e.g., disclosures of Eggers et al in U.S. Pat. Nos. 5,873,855; 5,888,198; 5,891,095; 6,024,733; 6,032,674; 6,066,134 and the companion patents cited therein). The Coblator™ system relies on the creation of a voltage difference between a plurality of closely spaced rod-like electrode elements in a distal working end and a return electrode on the instrument shaft. The Coblator™ system introduces an electrically conductive fluid such as isotonic saline into the physical gaps about a group of closely spaced active electrodes, and between the electrode group and the targeted tissue. The system applies electrical energy with a frequency of about 100 kHz and a voltage of about 100 to 300 V. The Coblator™ promotional materials explain that at high voltage levels, the electrically conductive fluid in the gaps between the closely spaced active electrodes is converted to steam and then into a plasma. The supposition underlying the Coblator™ is that the actual energy-tissue interaction produced by the system relates to charged particles in the plasma having sufficient energy to cause dissociation of molecular bonds within tissue structures that come into contact with the plasma. Based on this hypothesis, the accelerated charged particles have a very short range of travel, and the energy-tissue interaction causes molecular dissociation of tissue surfaces in contact with the plasma.
The types of ablation caused by conventional electrosurgical ablation and the ablation caused by the Coblator™ system share several common characteristics. While conventional ablations and the Coblator™ ablations are suitable for many procedures, both types of ablation are caused by intense energy delivery that boils a fluid (or water in tissue) to create an insulative steam layer which then is energized into a plasma in an interface with tissue.